1. Field of the Invention
The present invention is directed to the formation of a high density pattern for field emitter tips for field emission display (FED) devices. More specifically, the present invention is directed to a method of formation of a high density pattern for field emitter tips for FEDs using microspheres and/or nanospheres.
2. State of the Art
Field emission display (FED) devices are of the type of flat panel display in which a baseplate with a generally planar emitter substrate is juxtaposed to a faceplate with a substantially transparent display screen. The baseplate has a number of emitters formed in the emitter substrate that project from the emitter substrate towards the faceplate. The emitters are typically configured into discrete emitter sets in which the bases of the emitters of each emitter set are commonly connected. The baseplate also has an insulator layer formed on the emitter substrate and an extraction grid formed on the insulator layer. A number of holes are formed through the insulator layer and extraction grid in alignment with the emitters to open the emitters to the faceplate. In operation, a voltage differential is established between the extraction grid and the emitter to extract electrons from the emitters.
The display screen of the faceplate is coated with substantially transparent conductive material to form an anode, and the anode is coated with a cathodoluminescent layer. The anode draws the electrons extracted from the emitters through the extraction grid and the cathodoluminescent layer of material. As the electrons strike the cathodoluminescent layer, light emits from the impact site and travels through the anode and the glass panel of the display screen. The emitted light from each of the areas becomes all or part of a picture element.
In field emission displays, it is desirable to have a bright display at each picture element thereof in response to the emitted electrons from the emitters in the emitter set. The brightness at each picture element of a field emission display depends upon the density of the emitters in the emitter sets corresponding to each picture element. It is desirable to have a constant emitter density from one emitter set to another and from one area of the emitter set to another therein. It is further desirable to have the emitters spaced the same distance apart from other emitters in the same emitter set, and to have the emitters of each emitter set substantially the same size and overall shape.
One method for forming emitters is using photolithographic techniques. However, it is difficult to form conically shaped emitters using photolithographic techniques in high densities and over large areas using photolithographic techniques. Therefore, it is desirable to have an easily reproducible technique to form high densities of emitters over large areas for any desired size of field emission displays.
In another method of forming emitters for field emission displays, illustrated in U.S. Pat. No. 4,407,695, a large area lithographic mask is produced on the surface of a substrate by coating the substrate with a monolayer of colloidal particles such that the particles are fixed to the substrate. Depending upon the disposition technique used, the colloidal particles may be arranged on the surface of the substrate in either a random or ordered array. The array of particles can then be used as a lithographic mask and the random or ordered array can be transferred to the substrate using a suitable etching process. Alternately, the lithographic mask may be used as a deposition mask. The emitters are formed by randomly distributing a number of beads on a hard oxide layer that has been deposited over the emitter substrate.
As illustrated in U.S. Pat. No. 5,399,238, sharp sub-micron emitter tips for field emission displays are formed without requiring photolithography. Vapor deposition is used to randomly locate discrete nuclei to form a discontinuous etch mask. The nuclei are preferably non-polymerized with a relatively high melting point to assure that an ion etch produces pyramid shaped tips with a suitable enhancement factor. In one instance, an etch is applied to low work function material covered by randomly located nuclei to form emission tips in the low work function material. In another instance, an etch is applied to a base material covered by randomly located nuclei to form tips in the base material which are then coated with low work function material to form emission tips. Diamond is the preferred low work function material.
As illustrated in U.S. Pat. No. 5,676,853, a mask and method of making the mask comprises distributing a mixture of mask particles and spacer particles across a layer of material on a semiconductor wafer. The spacer particles space the mask particles apart from one another to prevent the mask particles from clustering together and to control the distance between mask particles. The mixture is preferably deposited onto the layer of material to form a substantially contiguous monolayer of mask and spacer particles across the surface of the wafer. The spacer particles are then selectively removed from the surface to the layer such that the mask particles remain on the layer in a pattern of spaced apart masked elements. The spacer and mask particles are preferably made from material with different etching selectivities that allow the spacer particles to be selectively etched from the wafer. In other instances, the physical differences may allow the spacer particles to be removed by selectively breaking a bond between the spacer particles and the surface layer, or by selectively evaporating, sublimating, or melting the spacer particles from the layer of material. The spacer particles and the underlying layer of material upon which the spacer particles are deposited are preferably made from materials that may be selectively etched without etching the mask particles. The spacer particles and the underlying layer of material may accordingly be etched in a single process step to form a desired pattern of island-like elements under the mask particles.
As illustrated in U.S. Pat. No. 5,510,156, a method is disclosed wherein the deposition of latex spheres on a sacrificial layer on a substrate, shrinking of the spheres, depositing a metal over the spheres, dissolving the spheres, etching the substrate through the openings formed by removing the spheres, removing the remaining metal, and depositing the desired microstructure material over the sacrificial layer are used to form a textured top surface of the sacrificial layer.
Illustrated in U.S. Pat. No. 5,695,658, a non-photolithographic, physical patterning process is described for the selective etching of a substrate. The process comprises electrostatically charging liquid droplets which are selectively etchable with respect to the substrate, dispersing the droplets onto the substrate in a pattern, and etching the substrate using the droplets as a mask.
In yet another instance, self-assembled polystyrene beads whose diameter can be arbitrarily reduced by reactive ion etching are used to produce a hole array on a silicon substrate which is subsequently filled with material. The beads may have a diameter to allow the formation of a nanostructure array. Alternately, latex beads may be used rather than polystyrene beads.
In another instance, micron and sub-micron holes are formed in field emitter displays which use microspheres to bring parallel beams of ultraviolet radiation into numerous foci on a photoresist which is used as a mask.
In all the described prior art processes, none provides a simple, nonphotolithographic process for the manufacture of emitters for a field emission display using a minimum of process steps wherein a high density of emitters in the emitter set is of substantially equal spacing from adjacent emitters and of substantially equal height. Therefore, a need exists for such a process for the forming of a high density of emitters in the emitter set for a field emission display.
The present invention is directed to a method of formation of a high density pattern for field emitter tips for FEDs using microspheres or nanospheres. The present invention includes a method of forming a pattern in a layer of material on a substrate, comprising providing a plurality of spheres, covering the layer on the substrate with the plurality of spheres to form a mask, reducing the diameter of at least one sphere of the plurality of spheres, etching the layer on the substrate using the at least one sphere having a reduced diameter as a mask, and etching the substrate.